I don't know anything about Tk. I haven't touched it ever. But I found a solution to this question in less than a minute. So all I have to say is RTFM.

use Tk;

my $mw = Tk::MainWindow->new( -title => "TEST");
  
$mw->bind( '<Control-KeyPress-a>', [sub {print "Button a has been pres
+sed\n"}] );
$mw->bind( '<Control-KeyRelease-a>', [sub {print "Button a has been re
+leased\n"}] );

MainLoop;
[download]

Update: Oh, I forgot about the keysym list. Well, doing a locate keysym showed up /usr/X11R6/include/X11/keysymdef.h. You may have to install a development package to get it (in Debian, it's x-dev).